The typical form front of the prior art serves to connect the shield of a tunnel excavator to the tunnel-lining form and defines the front of the space between the lining form and the tunnel wall to be filled with concrete.
It generally comprises an annular support positioned with clearance between the shield cover and the tunnel-lining form, but having a lipped inner sealing collar between it and the tunnel-lining form and an outer sealing collar between it and a shield extension of the tunnel excavator.
In the earlier form front the outer sealing collar formed a unit with the lipped inner collar. A lipped sealing collar fulfills its function best as a sealing member spanning a gap when relative motion in only one direction occurs between the lipped seal and the surface engaged thereby. As to the lipped sealing collar engaging the tunnel-lining form this is the case because the lipped sealing collar is drawn over the stationary tunnel-lining form which generally is a translatable or movable form in only one direction. With the outer sealing collar, however, which engages the shield, relative motion of the seal and the shield does occur in both axial directions. Thus the form front can move compared to the shield but also the shield can move with respect to form front. Hence when the outer seal is formed as a lip seal, the disadvantages of leaking and loosening due to bending over of the seal can occur, and increased wear must be expected.
Of course it is also known in practice with other form fronts to install a circular sealing collar as an outer seal which has a symmetrical cross section with respect to its central symmetry plane parallel to the form front plane and is positioned in a circular groove provided in the outer covering surface of the form front. One such seal with a symmetrical profile allows relative motion in two directions opposite to each other and is made tight by its own compressibility, which assumes an installation under compression, which with the form front of the kind set forth creates problems with set up and replacement of the seal.